Pharmacokinetic and biodistribution analysis of monoclonal antibodies: a comprehensive study of antibody biodistribution and partitioning coefficients in mice

The efficacy of therapeutic monoclonal antibodies (mAbs) often hinges on biodistribution to their site of action. However, traditional pharmacokinetic (PK) assessments - typically based on measuring plasma or total tissue concentrations - fail to reflect the interstitial concentrations that are most relevant for tissue targets. This study aimed to address this limitation by integrating experimentally determined vascular and interstitial volumes from tissues in SCID-beige mice with a comprehensive PK time-course and biodistribution analysis of four distinct anti-viral monoclonal antibodies (mAbs 1-4) with no endogenous mouse target. The biodistribution studies included 11 tissues, characterizing tissue and plasma concentrations over a 168-h time-course. Total and interstitial tissue concentrations were evaluated to better understand concentrations within the interstitial space compared to bulk tissue values. These data revealed significant tissue-specific partitioning, with fold-change analysis suggesting groupings correlating with capillary endothelium characteristics. A dynamic model was implemented for the estimation of antibody biodistribution coefficient (ABC) values at steady-state, partitioning ratio (PR) values at steady-state, and their associated equilibrium rate constants (t_1/2eq, t'_1/2eq) across 11 (ABC, t_1/2eq) and 7 tissues (PR, t'_1/2eq), respectively. Specifically, to understand non-binding, target-independent biodistribution, we combined data from mAbs 1, 2, and 3 to create a "typical mAb" (mAb 123) profile, from which these coefficients and ratios were derived. Analysis of mAb 4, a structurally similar IgG molecule with undesirable PK properties, enabled comparative insights into antibody distribution and kinetics. These studies provided a comprehensive dataset for understanding interstitial antibody PK, crucial for improving predictions of PK at the site-of-action and in vivo efficacy.